The identification of matter by detecting the spectrum of the electromagnetic radiation reflected from or transmitted through a sample is a well-developed technology. (In this specification and in the claims we use the term, "light" in its broad sense to refer to electromagnetic radiation of any wavelength, including ultraviolet light, visible light and infrared light.) A favorite device for identifying light spectrum is the Michelson interferometer, which was invented by Albert Michelson in 1891 which is still used extensively today. In many modern instruments, based on an improvement invented by Myron Block, the displacement of Michelson's moving mirror is monitored interferometrically along with the detected reflected radiation. Both signals are then analyzed by a computer, which performs a Fourier transform on the collected data to convert a matrix of intensity and displacement data into a spectrum chart of intensity vs wavelength, frequency or wave number. A particular technique, which has been utilized for many years, is the measurement with these spectrometers of diffuse reflectance and diffuse transmittance. When samples of a particular material are illuminated with radiation having a known broad band spectrum, usually in the infrared, visible or ultraviolet range, the sample will absorb and reflect the radiation in a manner that is unique to the particular material.
Integrating spheres have been used for diffuse reflectance spectroscopy since the 1920's. A well-known technique for making diffuse spectrographic measurements involves the use of integrating spheres and a Fourier transform infrared spectrometer. In these devices a necessarily small and rather flat sample is located on a small portion of the inside surface of a sphere having highly reflective diffuse inside surfaces. Infrared light from a broad-band source passes through a slow moving interferometer, illuminates the sample then reflects multiple times from the walls of the integrating sphere and a portion of the light is detected with a detector mounted in a portion of the wall of the sphere. The inside surface of the integrating sphere is rough and very highly reflecting at the wavelength of the illuminating light in order to provide the very large number of diffuse reflections. Systems may utilize a chopped beam so that the detector sees alternating signals from a reference path of source, interferometer, reference, and detector and a sample path of source, interferometer, sample and detector. Special treatments for powder samples and wet samples are usually required before the samples can be analyzed.
FIGS. 1A, 1B and 1C describe a prior art infrared spectrometer, which has been used commercially since the mid-1970's. See Griffiths, Fourier Transform Infrared Spectrometry, pages 194-197, published by Wiley-Interscience, New York, N.Y. In this device as shown in FIG. 1A, the displacement of scanning mirror 2 is measured with an HeNe based interferometer system consisting of HeNe laser 6, fixed mirror 4, scanning mirror 2 and laser detector 8. Infrared light beam from source 10 is collimated by mirrors 12 and 14 and split and joined again by beam splitter 16 after the divided beam has reflected off fixed mirror 4 and scanning mirror 2. The recombined beam is reflected off mirror 18 and either passes through chopper 20 or is reflected by it or absorbed by it as chopper 20 rotates. The passing portion, as shown in FIG. 1B, is reflected by mirrors 26 and 28 and illuminates sample 22 in integrating sphere 24. Light not absorbed by the sample is reflected multiple times from the diffusely reflecting inside surface of integrating sphere 24 and a portion of the beam is detected by detector 26. The reflecting portion, as shown in FIG. 1C, is reflected further by mirror 30 and illuminates reference 32. Light not absorbed by reference 32 is reflected multiple times from the diffusely reflecting inside surface of integrating sphere 24 and a portion of the beam is detected by detector 26. When the beam is absorbed by chopper 20, the signal from detector 26 represents a zero energy signal and is subtracted from the sample and reference signals. The signals are analyzed by a computer not shown. The signal from laser detector 8 provides a zero reference twice each time the scanning mirror is displaced by a distance equal to the wavelength of the laser beam. These zero signals are utilized by the computer as a timer to read the infrared signal from detector 26 so that the matrix of collected data is intensity vs mirror displacement. From this array of data the computer calculates the ratio of sample to reference after subtracting the zero signal from each and from the resulting matrix of data, the computer performs a Fourier transform to develop an absorption spectrum for the sample.
The device shown in FIGS. 1A, 1B and 1C and similar devices are very useful in many applications; however, for many other applications a low signal to noise ratio (SNR) has been a problem. Therefore, other diffuse reflectance techniques utilizing optical configurations with better SNR and not involving an integrating sphere have for the most part preempted the systems using integrating spheres. One of these devices uses a paraboloid to focus the beam from the interferometer on to the sample and the light from the sample is collected by another paraboloid and focused onto the detector. This device permits about 15 percent of the light from the interferometer to reach the detector and provides a much better signal to noise ratio.
The prior art of detecting small quantity contaminants, especially from wet processes includes a technique known as attenuated total reflectance (ATR). A solvent containing a contaminant is deposited on IR transparent crystals contained in the trough of a holder and the solvent is allowed to evaporate leaving the contaminant deposited on the transparent crystals. The infrared spectrum is recorded and the data quantified. The ATR technique is deficient in several respects. Errors result if the solution does not completely cover the crystals and when material gets deposited on the walls of the trough. Cleaning the holder, trough and the crystals can be difficult, especially if the crystals are scratched. And the crystals are expensive. Sample preparation can be time consuming. Errors from mishandling samples can result from the many steps required. These problems are usually even more serious in field-testing as co pared to laboratory testing.
What is needed is a better device and method for providing quantitative spectroscopic measurements of very small samples such as low levels of non-volatile contaminants. The need is especially great for field portable and production process measurements.